1. Field of the Invention
This invention relates generally to vision correction by spectacles and eyeglasses and, more specifically, to eye contact type ophthalmic lenses.
2. Description of the Prior Art
Contact lenses are well known in the art and have enjoyed commercial success for some time. Early disclosures of contact lenses are typified by U.S. Pat. Nos. 722,059 and 1,869,366 which describe scleral lenses made of glass. U.S. Pat. Nos. Re. 25,286 and Re. 29,229 are exemplary of disclosures relating to corneal contact lenses.
Much effort has been directed to improving the performance and the physical comfort of contact lenses. Improvements generally relate to materials and lens shape.
The use of silica filled siloxane elastomers as lens materials has been proposed (U.S. Pat. No. 3,228,741). Such elastomers, commonly known as silicone rubbers, are desirable contact lens materials because of their oxygen permeability and their well known bio-compatability. However, the difficulties of designing a satisfactory lens from a silicone rubber material are well known. Silicone rubber has a high modulous of elasticity causing lenses made from silicone rubber to tend to adhere or to "suction" to the eye upon blinking. This phenomenon is often referred to as the "suction cup effect."
Many lens designs have been proposed for use with both silicone rubber and other materials. Scleral contact lenses which arch over the cornea and are supported by the sclera are known (U.S. Pat. Nos. 2,129,305; 2,129,304; 2,178,873; 2,196,066; 2,438,743; 2,330,837; 3,781,096 and 3,937,566).
A scleral lens having an inner surface formed from two spherical sections with each section having a different radius of curvature and spherical center has been disclosed (U.S. Pat. No. 1,921,972). According to that disclosure the sharp edge where the two sections meet is smoothed by forming a third cut which can have an infinite radius of curvature, i.e., a conical section. This design does not take into consideration the topography of the individual eyeball and, as a result, does not achieve either uniform tearfilm clearance or uniform pressure of the lens against the eye at all points from the edge of the lens to the center of the optical zone.
Flush fitting semi-scleral contact lenses which are in total contact with the cornea and incapable of easy movement relative to the cornea have been disclosed (U.S. Pat. No. Re. 29,229).
The same publication also discloses semi-scleral lenses which arch over the cornea. Such lenses are often given to excessive movement on the eye due to gravity and eyelid drag. Such movement is commonly referred to as lag and causes variable vision on blinking. Variable vision can be distracting when it is excessive.
A lens design which includes a central portion of the lens resting on the cornea and a supporting flange or border resting on the scleral portion of the eye is also known. Such a design is known to be difficult to fit accurately, and it is observed that the flange may seal off the corneal area from the flow of eye fluids. A proposed solution to this problem (U.S. Pat. No. 2,510,438) is a corneal lens which contacts the central cornea and which has a slightly raised marginal portion.
A lens shape which is useful with a variety of materials is desirable. That is, a design which can be used with hard lens materials such as polymethylmethacrylate (PMMA) as well as soft materials such as hydratable gels, e.g. hydroxyethylmethacrylate (HEMA) and collagenous materials, and silicone rubbers would be useful.
The use of inner surface lens shapes which include offset and conical aspheric curves wherein the aspheric curve and the optical zone inner curve are tangent where they meet is also known (Contact Lens Correction by Bier and Lowther, Butterworths, London and Boston, 1977). An offset aspheric curve is one having a spherical center not on the centerline of the optical zone. A conical aspheric zone is one having an infinite radius of curvature. Such lens designs can result in uneven tearfilm clearance at various locations between the periphery of the lens and the center of its optical zone, especially when the lens is not shaped to match the topography of an individual eyeball.
Lenses which are shaped to take into consideration the topography of the individual eyeball are described in commonly assigned copending patent applications U.S. Ser. No. 919,651 filed June 28, 1978 and U.S. Ser. No. 945,728 filed Sept. 25, 1978. U.S. Ser. No. 919,651 describes semi-scleral lenses which have an optical zone shaped relative to an individual cornea and a conical peripheral zone which is tangent to the optical zone and which bridges the limbus.
U.S. Ser. No. 945,728 describes a lens in which the optical zone is shaped relative to an individual cornea and in which at least two peripheral zones have offset centers of curvature.
These lens designs provide a tearfilm clearance of improved uniformity and a reduced incidence of "suctioning." However, there remains a need for substantially complete tearfilm uniformity and a substantial absence of the "suctioning" phenomenon along with substantially uniform loading of the lens against the cornea.
Other significant problems exist in the manufacturing area. It is a notable problem that previously known lens designs usually have a peripheral zone which tapers in cross-section toward the outer edge of the lens. During manufacture, the lens is normally cut to the desired chordal diameter and then smoothed around its edge. The tapering cross-section often requires a different size edging tool for each increment in diameter. Such a requirement is both labor intensive and expensive.
It is a further manufacturing problem that some lens materials, such as silicone rubbers, which are used to form lenses by molding tend to change size, primarily by shrinking, after removal from the mold. Such shrinking can frustrate the most careful efforts to shape a lens to correspond to an individual eye topography.
Yet another manufacturing problem exists in that different lens corrective powers have different optical zone thicknesses and, therefore, different masses. The lens performance on the eye can vary with mass, and a different lens performance can be observed for lenses having the same diameter but different optical correction.